1. Field of the Invention
The present invention relates generally to control systems for automatic transmissions employed in vehicles, and more particularly, to a system for controlling an automatic transmission provided in a vehicle to have a speed position selected automatically in accordance with the operating condition of an engine employed in the vehicle and the travelling condition of the vehicle.
2. Description of the Prior Art
There has been commonly used in vehicles an automatic transmission which comprises a torque converter coupled with an output shaft of an engine, a power transmitting gear arrangement including a planetary gear mechanism connected to an output shaft of the torque converter, a speed change mechanism including clutch and brake elements and operative to switch over power transmitting paths from one to another in the power transmitting gear arrangement so that speed change is carried out, a hydraulic servo control device provided in an oil hydraulic control circuit for causing the speed change mechanism to work, and a valve arrangement for controlling an operation oil to be supplied to and drained from the speed change mechanism and the hydraulic servo control device. The valve arrangement is operative to cause the hydraulic servo control device to act on the speed change mechanism in response to the operating condition of the engine and the travelling condition of the vehicle, so that the speed change mechanism works to switch over the power transmitting paths from one to another in the power transmitting gear arrangement under the control by the hydraulic servo control device.
For such an automatic transmission employed in the vehicle, it is desired that the shock of great degree, which may arise on the occasion of, for example, a kick-down operation in which the speed is shifted down in the power transmitting gear arrangement, is avoided and in addition a superior responsibility for speed changes is obtained. For fulfilling these desires, it is required that the timing of each speed change is adjusted appropriately in accordance with the operating condition of the engine and the travelling condition of the vehicle.
In view of this, for example, as disclosed in the Japanese patent application published before examination under publication number 59/183160, there has been proposed a control device for an automatic transmission in which a control valve mechanism for controlling the velocity of an oil flowing into or from a hydraulic servo control device which has an applying chamber and a releasing chamber to each of which an oil is supplied and is operative to cause a speed change mechanism comprising friction elements to work for changing a power transmitting path in a power transmitting gear arrangement, so as to control the acting speed of the hydraulic servo control device, is provided with the intention of adjusting the timing of each speed change caused by the speed change mechanism in response to the operating condition of an engine. However, although the acting speed of the hydraulic servo control device is influenced by the presence or absence of oil pressure in the applying chamber, such a fact has not been taken into consideration in the previously proposed control device. Therefore, the previously proposed control device has failed to realize the anticipated result.